The present invention relates to plate-like composite ferrite fine particles for magnetic recording which have a large magnetization, an appropriate coersive force, a large anisotropy field, a particle size of less than 0.1 .mu.m and an excellent temperature stability, and a process for producing such plate-like composite ferrite fine particles.
As described, for example, in Japanese Patent Application Laid-Open (KOKAI) No. 55-86103 (1980), ferromagnetic non-acicular particles have recently been demanded as a magnetic recording material, in particular, as a perpendicular magnetic recording material.
Generally, plate-like ferrite containing Ba particles are known as ferromagnetic non-acicular particles.
A method of autoclaving an aqueous alkaline suspension containing Ba ions and Fe.sup.3+ by using an autoclave as a reaction apparatus (this method is hereinunder referred to as "autoclaving method") is hitherto known as a process for producing plate-like ferrite.
Plate-like ferrite fine particles for magnetic recording are required to have as small a particle size as possible, to have an appropriate coercive force, a large magnetization and a large anisotropy field as the magnetic properties, and to be so excellent in the temperature dependence of the coercive force(temperature stability)scarcely varies or has a tendency to lower with the rise of temperature. This will be explained in more detail in the following.
Firstly, the particle size of plate-like ferrite fine particles must be as small as possible, as is seen from FIG. 3 on page 27, lines 23 to 29 in the REPORTS OF TECHNICAL RESEARCHES OF THE INSTITUTE 0F ELECTRONICS AND COMMUNICATION ENGINEERS OF JAPAN MR 81-11. FIG. 3 shows the relationship between the particle size of Co-doped acicular maghemite particles and the noise level, and it is clear from FIG. 3 that the particle size becomes smaller, the more the noise level linearly lowers. This relationship also holds for plate-like ferrite containing Ba particles.
Secondly, with respect to the magnetic properties, the plate-like ferrite containing Ba fine particles are generally required to have a coercive force of about 300 to 2,500 Oe, and in order to reduce the coercive force of the plate-like ferrite containing Ba particles produced in the autoclaving method to an appropriate coercive force, a method of substituting a part of Fe.sup.3+ in the ferrite by Co.sup.2+, Ti.sup.4+ or the ions of a divalent metal M.sup.2+ such as Mn and Zn has been proposed.
The magnetization of plate-like ferrite containing Ba particles must be as large as possible, as is described in Japanese Patent Application Laid-Open (KOKAI) No. 56-149328 (1981), "... the magneto plumbite ferrite which is used as a magnetic recording material is required to have the greatest possible saturation magnetization..."
The coercive force of plate-like ferrite containing Ba particles generally has a tendency of increasing with the rise of temperature, as is seen from FIG. 4 on page 1123 of IEEE TRANSACTIONS ON MAGNETICS MAG-18 No. 6. At the time of recording and reproduction, the temperatures of a magnetic head and a magnetic medium are raised with the reciprocal friction, so that the writing capacity for recording of the magnetic head is lowered, while the coercive force of the medium containing the plate-like ferrite containing Ba particles as magnetic particles is increased with the rise of temperature, resulting in the drop of output or deterioration of the overwrite characteristic. It is therefore necessary in order to enable record-writing in spite of the lowered writing capacity of the magnetic head that the coercive force of the medium scarcely varies or rather has a tendency to lower with the rise of temperature. Namely, it is necessary that the coercive force of plate-like ferrite containing Ba particles scarcely varies or rather has a tendency to lower with the rise of temperature. It is also pointed out, however, that the lowering of the coercive force more than necessary, is unfavorable from the point of view of recording stability.
Plate-like ferrite containing Ba particles are also required to have a large anisotropy field so as to enable high-density recording without lowering the output even in a high-frequency band. This fact is described, for example, on pages 67 to 68 of HIGH-DENSITY MEMORY TECHNIQUE AND MATERIALS (1984), published by K.K. CMC, "FIGS. 2, 3 and 12 show the characteristics between the output and the wavelength obtained by recording/reproducing with a ring head by using a Co-Cr single-layer medium having a large perpendicular anisotropy field (Hk). An excellent high-density recording characteristic such as D.sub.50 =135 KBPI is obtained ...."
Plate-like ferrite fine particles which have as small a particle size as possible, a large magnetization, an appropriate coercive force, a large anisotropy field and an excellent temperature stability are now in the strongest demand. In the above-described autoclaving method, various kinds of ferrite particles precipitate depending on the selected reaction conditions. The precipitated particles ordinarily have a shape of a hexagonal plate, and the particle properties such as particle size distribution and average particle diameter, and the magnetic properties such as coercive force, magnetization and temperature stability are different depending upon the conditions under which the ferrite particles are produced.
For example, in plate-like composite ferrite fine particles containing Co.sup.2+ -Ti.sup.4+ obtained by producing plate-like Ba ferrite fine particles containing Co.sup.2+ -Ti.sup.4+ by autoclaving method while substituting a part of Fe.sup.3+ in the ferrite by Co.sup.2+ and Ti.sup.4+ in order to reduce the coercive force to an appropriate coercive force and heat-treating the thus-obtained fine particles, the coercive force reducing-effect of Co.sup.2+ -Ti.sup.4+ is large, so that the necessary amount of Co.sup.2+ and Ti.sup.4+ added to appropriately control the coercive force is so small as not to greatly lower the magnetization, which is maintained at a comparatively large value such as about 50 to 60 emu/g. However, the temperature dependence of the coercive force is +2.5 Oe/.degree. C. to 6.0 Oe/.degree. C., and the coercive force has a tendency of increasing with the rise of temperature, as described above. This phenomenon is inferred from FIG. 1 on page 1459 of JOURNAL OF MAGNETISM AND MAGNETIC MATERIALS Nos. 15 to 18 (1980).
In the case of the plate-like composite ferrite fine particles containing Ni.sup.2+ -Ti.sup.4+ produced by autoclaving method while substituting a part of Fe.sup.3+ in the ferrite by equimolar Ni.sup.2+ and Ti.sup.4+ in order to reduce the coercive force to an appropriate coercive force, the particle size is not less than 0.1 .mu.m, and in the plate-like composite ferrite fine particles containing equimolar Ni.sup.2+ -Ti.sup.4+ obtained by heat-treating the thus obtained fine particles, the coercive force reducing-effect of Ni.sup.2+ -Ti.sup.4+ is so small that a large amount of Ni.sup.2+ and Ti.sup.4+ must be added for appropriate control of the coercive force, thereby reducing the magnetization to as low as about 47 emu/g at most. The temperature dependence of the coercive force is comparatively superior to that of the plate-like ferrite fine particles containing Co.sup.2+ -Ti.sup.4+, as inferred from FIG. 1 on page 1459 of JOURNAL OF MAGNETISM AND MAGNETIC MATERIALS Nos. 15 to 18 (1980), but the temperature dependence of the coercive force of about 1.0 to 3.0 Oe/.degree. C. cannot still be said to be satisfactory.
As a method of improving the temperature stability of plate-like composite ferrite fine particles containing elements such as Co.sup.2+ -Ti.sup.4+ for reducing the coercive force, methods disclosed in, for example, Japanese Patent Application (KOKAI) Laid-Open Nos. 61-152003 (1986) and 62-132732 (1987) are conventionally known.
The method disclosed in Japanese Patent Application (KOKAI) Laid-Open No. 61-152003 (1986) is a method of heat-treating the plate-like composite ferrite fine particles containing elements such as Co.sup.2+ -Ti.sup.4+ for reducing the coercive force at a temperature of 300.degree. to 700.degree. C. in a reducing atmosphere. This method is disadvantageous in that the coercive force after heat-treating is increased to more than double the value before heat-treating, thereby making the appropriate control of the coercive force difficult.
The method disclosed in Japanese Patent Application (KOKAI) Laid-Open No. 62-132732 (1987) in a method of making the particle shapes produced uniform such that the average particle diameter is not more than 1.0 .mu.m, the thickness in the direction of the c-axis is not more than 0.2 .mu.m and a plate ratio (average diameter of plate surfaces/thickness in the direction of c-axis) is not less than 5. This method is disadvantageous in that the particle shape is regulated in order to improve the temperature stability.
As a method of enhancing the magnetization of plate-like composite ferrite containing Ba particles, for example, methods of modifying the surfaces of the plate-like composite ferrite containing Ba particles by spinel ferrite (disclosed in Japanese Patent Application Laid-Open (KOKAI) Nos. 60-255628 (1985), 60-255629 (1985), 62-139121 (1987), 62-139122 (1987), 62-139123 (1987) and 62-139124 (1987)) is conventionally known. An anisotropy field of the plate-like ferrite fine particles produced by these proposed methods is as small as about 2 to 3 KOe. Further by these proposed methods, it is impossible to obtain well-balanced plate-like ferrite fine particles having an average particle diameter of less than 0.1 .mu.m, a large magnetization, an appropriate coercive force and an excellent temperature stability.
The establishment of a method of producing plate-like Ba ferrite fine particles having as small a particle size as possible, an appropriate coercive force, a large magnetization, a large anisotropy field and an excellent temperature stability without any regulation in the particle shape has therefore been strongly demanded.
As a result of various studies on a process for producing plate-like ferrite particles having as small a particle size as possible, an appropriate coercive force, a large magnetization, a large anisotropy field, and an excellent temperature stability that the coercive force scarcely varies with-the rise of temperature without any regulation in the particle shape, it has been found that by autoclaving a suspension of an alkaline iron (III) hydroxide containing 0.125 to 0.25 atom of Ba ions based on 1 atom of Fe.sup.3+ in the temperature range of 100.degree. to 300.degree. C., while adding in advance 2 to 5.0 atomic% of a Ti compound based on Fe.sup.3+ and 1.2.ltoreq.Ni/Ti.ltoreq.4 by molar ratio of a Ni compound to the suspension of the alkaline iron (III) hydroxide to produce the plate-like composite ferrite containing Ba fine particles, suspending the thus obtained fine particles in an aqueous solution containing zinc and having a pH of 4.0 to 12.0, filtering out the fine particles with a Zn hydroxide precipitated on the surfaces thereof, washing them with water, drying them and calcining (heat-treating) them in the temperature range of 600.degree. to 900.degree. C., the thus obtained plate-like composite ferrite containing Ba particles have a concentration gradient of Zn with a specified percentage through particle in the form of a solid solution, and have an average particle diameter of 0.01 .mu.m to 0.08 .mu.m, an appropriate coercive force, a large magnetization and a large anisotropy field, wherein the coercive force thereof scarcely varies with the rise in temperature in the temperature range of -20.degree. C. to 120.degree. C. The present invention has been achieved on the basis of this finding.